An important metrology task for applications such as semiconductor photolithography is offset measurement (also known as overlay measurement). Offset measurement is the measurement of a lateral misalignment (i.e., the offset) of one layer relative to another layer. Typically the two layers are parallel to each other and in proximity to each other. For example, one layer can be a patterned metal and/or semiconductor first layer on a substrate and the other layer can be a pattered photoresist second layer deposited on the first layer. Measurement of the alignment of features of the photoresist layer relative to features in the first layer is an important process metric.
Accordingly, many ways have been devised to perform offset measurements. Both imaging and non-imaging approaches for offset measurement have been developed. Imaging approaches are conceptually straightforward, since they are based on analysis of a “picture” directly showing the alignment of the two layers. For example, box-in-box or line-in-line alignment marks are commonly used in the two layers. However, imaging approaches are sensitive to vibrations which can blur the pictures. Such vibrations are difficult to eliminate from a commercial semiconductor processing line. Thus non-imaging approaches, which can be less sensitive to vibration, are presently being investigated.
Many distinct non-imaging optical overlay measurement approaches have been developed to date. For example, scatterometry (or diffraction) approaches are based on illumination of gratings in the two layers and measurement of optical signals from the gratings. Analysis of the measured optical signals can be used to determine the offset. It is important to distinguish methods which are based on optical diffraction from methods which only work in the absence of diffraction (e.g., when the features being probed are much larger than an optical wavelength). U.S. Pat. No. 4,757,207 by Chappelow et al. is an example of such a non-diffraction approach. Thus in the following considerations, “grating” refers to a periodic structure having a period on the order of an optical wavelength or smaller.
Generally, it is important to determine both the sign and the magnitude of the offset. Determination of the sign of the offset via scatterometry is an issue which is appreciated by some art workers, but not by others.
In U.S. Pat. No. 4,200,395 by Smith et al., diffracted beams in equal and opposite orders (e.g., +1 and −1 orders) are measured. This approach provides enough information to resolve potential sign ambiguities. However, it is often preferable to make use of the zero order diffracted beam, since the zero order beam is often the most intense diffracted beam and provides the best signal. Another disadvantage of the approach of Smith et al. is that it is difficult to perform the required measurements over a wide wavelength range because the angle at which a diffracted beam is emitted depends on wavelength for all orders other than zero order.
In U.S. Pat. No. 6,699,624 by Niu et al., overlay measurement via scatterometry is considered, but there is no discussion of how sign information is obtained. Modeled results are presented including sign information. This sign information may possibly be obtained from analysis of optical signals from test patterns having multiple grating orientations. For example, these orientations can be horizontal (0°), vertical (90°), +45° and −45°. In this work, the nominal alignment (i.e., the alignment having zero offset) is when the two gratings are aligned. More specifically, the lines in the second grating are aligned with either the lines in the first grating or the spaces in the first grating. The use of multiple grating orientations to resolve the sign ambiguity (e.g., 4 grating orientations to determine 2 orthogonal offset magnitudes and signs) is undesirable, since it requires more chip area than an approach which only requires one grating orientation.
Sign determination is considered more explicitly in U.S. Pat. No. 6,772,084 by Bischoff et al. In this work, the preferred nominal alignment is when the two gratings are offset by about a quarter period. By selecting this nominal alignment, the sensitivity of the zeroth order reflection to changes in the offset is improved, and sign information is more readily available from the zeroth order reflection signal. In this work, the symmetry that often leads to equal zeroth order diffraction signals for positive and negative offsets having the same magnitude is broken by making the compound grating formed by the two gratings asymmetric at zero offset. However, the use of an asymmetric nominal alignment can lead to complications in practice that would be avoided if the nominal grating alignment were symmetric. Gratings having a nominal alignment offset of about a quarter period are also considered by Yang et al. in an article entitled “A novel diffraction based spectroscopic method for overlay metrology” in Proc. SPIE v5038 pp. 200-207.
An approach where two stacked gratings having equal and opposite offsets at nominal alignment is considered by Huang et al. in an article entitled “Symmetry-Based overlay metrology” in Proc. SPIE v5038 pp. 126-137. The difference in reflectance between the two stacked gratings is a measure of both the sign and magnitude of the offset. For sufficiently small offsets, this reflectance difference is approximately a linear function of offset. This approach requires provision of sufficient chip area for two stacked gratings. It would be preferable to obtain the sign and magnitude of the offset with a single stacked grating.
Asymmetric gratings for scatterometry are also considered in US 2002/0158193 by Sezginer et al. In this work, the first and/or second gratings are individually asymmetric in order to further reduce the offset sign ambiguity. Detailed modeling is performed in order to determine the offset from the measured signals. However, the use of asymmetric gratings in this approach can lead to significant complications in the calculations for determining the offset. Since rapid results are frequently required in a production environment, it is desirable to reduce modeling time, and even more desirable to eliminate modeling entirely.
Accordingly, it would be an advance in the art to provide scatterometry for determining both sign and magnitude of an offset that provides rapid results by reducing modeling calculations. Another advance in the art would be to provide scatterometry for determination of the sign and magnitude of the offset from a nominally symmetric arrangement of two overlapping gratings.